Quadrature-witness readout for backscatter mitigation in gravitational-wave detectors dominated by backaction

Disturbances in gravitational wave (GW) observational data are often caused by nonstationary noise in the detector itself, such as backscattering of laser stray light into the signal field. Unlike GW signals, nonstationary noise can appear in both the GW-signal quadrature and the orthogonal quadrature, which is usually not measured. Simultaneous sensing of this orthogonal quadrature provides a witness channel that can be used to reconstruct the disturbance in the signal quadrature enabling subtraction of nonstationary noise. Here, we present the concept of quadrature witness that is compatible with frequency-dependent squeezing, which is already used to simultaneously reduce photon shot noise and photon radiation-pressure noise. We demonstrate that implementing this approach in a GW detector could reduce noise caused by loud backscatter events, thereby improving the overall sensitivity and robustness of GW observatories.